1. Field of the Invention
The present invention relates to a charging device for a radio communication device, and particularly to a charging device for charging, in an electrically noncontacting state, a radio communication device such as a storage-battery-equipped radiotelephone.
2. Description of the Related Art
Charging devices have been suggested recent years for charging chargeable storage batteries installed in radio communication devices such as radiotelephones that are capable of charging without being electrically connected to the radiotelephone. One example of such a device is a noncontacting charging device disclosed in Japanese Patent Laid-open 280631/90. In this charging device, a battery charger is provided with a primary coil to which an alternating current signal is supplied, while a radiotelephone is provided with a secondary coil which couples electromagnetically with the primary coil and which is connected to a charging circuit for a storage battery. By bringing the radiotelephone into proximity with the charger and supplying the primary coil with alternating current of prescribed voltage and frequency, an induced electromotive force is generated in the secondary coil to serve to charge the storage battery of the radiotelephone.
In this charging device of the prior art, as the mutual induction coefficient between the primary and secondary coils is increased to raise an efficiency of an electric power supply to the storage battery of the radiotelephone, the electromagnetic attraction working between the two coils increase nearly in proportion to the mutual induction coefficient. This attractive force, however, adversely makes it difficult to lift lightly the radiotelephone from charger when it is necessary to make a call or to receive an incoming call during charging.
In such a case, charging is stopped in a charger of the prior art by pulling the power cord of an external power source connected to the charger from the commercial power socket. This necessity to pull the cord when lifting the radiotelephone from the charger is a nuisance, particularly when the radio telephone must be picked up quickly to receive an incoming call while using the charging device. However, if the power of the charger is decreased in order to weaken the electromagnetic attraction between the coils to circumvent this problem, there is the problem that power supplied to the storage battery of the radiotelephone also decreases, causing charging efficiency to drop, and making rapid charging impossible.